This invention relates generally to an electronic timepiece which is highly energy efficient and more particularly to an electronic timepiece which adjusts its voltage supply level to match the condition of operation. In recent years, the performance of a lithium battery has been improved and designers are beginning to use these batteries in an electronic timepiece. Also, because of the increasing price of silver, a lithium battery for use in a timepiece is attracting attention.
A lithium battery usually outputs a 2.8 to 3.0 voltage level, and has capacity for use in a timepiece of 60 to 100 milliampere hours at three volts. However, with regard to C-MOS integrated circuits for use in a wristwatch, it is generally known that the integrated circuit operates satisfactorily at 1.5 volts. Energy is wasted when the circuits operate from the three volt level of a lithium battery. Battery life is extended by operating with a half voltage, that is, approximately 1.5 volts, by using a circuit in conjunction with the lithium battery which switches the connection of two capacitors so they are alternately in series or in parallel.
Thus, the lithium battery is advantageously used at half voltage, and because of its low self-discharge rate characteristic, a wristwatch using a lithium battery can have a life of 5 to 7 years without battery change. However, in actual practice of using a lithium battery for a wristwatch, there is a problem in that the internal resistance of the lithium battery is very large. This is particularly true regarding a flat and small lithium battery as would be desirable for a wristwatch. Therefore, such a battery is not suitable for a wristwatch having heavy load circuits, for example, a lamp or an alarm circuit because of a significant internal voltage drop.
What is needed is an electronic timepiece having a power source which provides a stable voltage output during normal operation as well as during periods of heavy load.